From the days of the ‘Challenger’ expedition, biological oceanographers had relied on intuition that the extreme physical conditions and impoverished nutritional input could be tolerated only by a small number of specialized forms of life. This concept was overturned in the 1960s after examination of bottom samples obtained by Howard Sanders and Robert Hessler at the Woods Hole Oceanographic Institution using the fine-meshed epibenthic sled (see Chapter 5) uncovered an astonishing richness in species of the smaller animals dwelling in the deep-sea sediments (Hessler & Sanders, 1967; Sanders & Hessler, 1969). These workers showed that the trend of decreasing diversity with increasing depth in species caught in relatively coarse meshed deep-sea trawls was an artefact resulting from the decreased density of individual animals and the small number of samples taken at great depths.

Also, such high species richness had not been predicted by theoretical ecologists. Environments that are seemingly spatial and temporally uniform, with few and low-grade nutritional sources, were thought to tend towards an equilibrium with just a few competing species (Hutchin-son, 1953). We shall examine the literature that has attempted to explain this apparent anomaly. But first we must define the concept so that its measurement becomes tractable.

MEASUREMENT OF DIVERSITY

The term diversity is commonly used as a synonym for species richness, the number of species present in the community.